The following invention relates to a method and a construction for TFEL panels utilizing a solid filler material within the sealed cavity housing the active TFEL components.
TFEL panels are extremely sensitive to moisture. If such panels are operated without a protective cover plate in an environment containing moisture, the thin films making up the AC TFEL device tend to delaminate, leaving dark regions in the display area. This problem is progressive as long as the panel is operated in the moist environment and it is irreversible. If operated long enough in such an environment, the entire active area becomes dark.
Previous solutions to this problem have involved the encapsulation of the active TFEL area of the display with a cavitated cover plate and the filling of the intervening cavity with silicone oil. This technique is shown in the U.S. Pat. Nos. to Kawaguchi et al. 4,213,074, Endo et al. 4,446,399 and Kawaguchi et al. 4,447,757.
The silicone oil utilized in the aforementioned patents does not provide moisture protection but avoids mechanical problems associated with the cavity being filled with gas (air) or being evacuated. The oil also promotes self-healing for point defects which are small short circuits between the phosphor and one of the electrode layers. The moisture protection function is provided by the rear cover plate and a perimeter adhesive seal. If this seal is broken, or is not intact as a moisture barrier, the moisture-induced degradation of the panel will proceed. Mechanical problems associated with gas-filled or evacuated cavities include the fracturing of the cover plate or substrate at high or low ambient pressures (such as high altitude or submarine environments) and excessive acoustical vibration resulting from the piezoelectric effect in the ZnS phosphor layer of the device. Other problems which may be encountered, if the panels are installed in aircraft, include the actual shattering of the glass due to mechanical vibration or stress. In the event of a break in the rear cover plate the silicone oil in oil-filled devices can leak out and damage the electrical components below the panel.